1. Field of the Invention
The invention relates to an antenna, more particularly to a printed circuit board (PCB) antenna.
2. Description of the Related Art
The invention relates to an antenna, more particularly to a printed circuit board (PCB) antenna.
2. Description of the Related Art
With reference to FIGS. 1 and 2, a PCB antenna 30 commonly used in a small wireless device, such as a wireless USB device, has an unbalanced structure. Examples of the unbalanced antenna include a monopole antenna, a Planar Inverted F Antenna, etc. These types of antenna create image currents on the ground plane 32 of the PCB 31. These image currents will then create standing waves at the ground plane 32 and then can easily create problems such as side lobes and nulls in the radiation pattern of the antenna 30. The problems arise from the designs and structures of the products employing the antenna 30. In particular, the components on the PCB 31 are not standardized and the size of the ground plane 32 may vary. When the length of the ground plane 32 is less than one quarter wavelength of the operating frequency, the antenna 30 can have a good omnidirectional radiation pattern commonly seen in dipole antennas.
However, when the length of the PCB 31 is increased to accommodate more components, the length of the ground plane 32 usually exceeds one quarter wavelength of the operating frequency of the antenna 30. This then causes the image currents on the ground plane 32 to present standing waves and results in the nulls 33. The radiation direction for the antenna 30 will be pointed towards the PCB 31, causing the radiation pattern of the antenna 30 to have lots of side lobes and nulls. This not only affects the effectiveness of sending and receiving signals, but may also cause some electromagnetic interference to electrical components on the PCB 31.